Measurement of dielectrics



AJune r11, 1940. A. R. ocslLvlE 2,203,8.59

YMEASUREMENT OF DIELECTRIQS Filed Jan. 28, 1939 nventor atente'd June 1l, 1940 Allan B. Ogilvie, Oaklyn, N. J., allignor to Badin Corporation of America, a corporation of Delaappuosuon'unuary as, 193s, serial No. 25am claims. r(ci. 11s-'assi This invention relates to testing devices and riorerparticularly to an improved method of and neans for quickly and accurately comparing or letermining certain characteristics of samples f a dielectric material such as sheets of mica, or he like; and the objects of this invention are:y irst, to provide means for accurately indicating ,he variations in thickness and dielectric conatant of a number of sheets of dielectric material; second, to provide a device which will give an iccurate indication of the ultimate capacity which vill be obtained by evaporating metal on opposite iurfaces of a sheet of dielectric material; third, to describe an improved method of testing and grad-` ing mica for use as a dielectric in capacitors; fourth, to provide a device which, in testing a dielectric, eectiveiy reproduces a condition which will prevail in the subsequent use of the dielectric. This invention will be better understood from the following description when considered in connection with the accompanying drawing which represents an embodiment of this invention.

In the manufacture of small fixed capacitors, the natural variations in the thickness, the nature of the surface, and the dielectric constant of the insulating material have made it impossible tor accurately predetermine the resultant capacity. This results in a large percentage of rejects, especially when it is .necessary to maintain the ulti mate capacity within narrow limits.

Small fixed capacitors of high efficiency arel usually constructed of alternate layers of mica and copper or lea'd foil. Due to the fact that mica is a natural'mineral product and not a manufactured product, it is'impossible to obtain mica sheets of uniformthickness. Itis, therefore, necessary to measure and select the mica according to the desired use.y While the device to be hereinafter described is particularly adapted to the measurement of mica dielectric. it ls evident Athat it may also be utilized to test or grade similar dielectrics such as paper, or the like.

Manufacturers recently started to make capacitors, which have a relatively large capacitance and small size, by the process silver, or a similar metal, on opposite surfaces of thin sheets of mica. It becameevident that the thickness and the dielectric constant of the mica Y employed would have to be absolutely uniform to insure the economical production of the new capacitors. Since the effective dielectric area of the capacitor can be held to very close tolerances by utilizing masks at the time the silver is evap-` orated on to the surfaces of the mica, it became necessary to provide a system for testing the characteristics of the micabefore applying the metal coating in order to control the value of the ,ultimate capacity.

The mica commercially used 'in capacitor manufacture is generally less than .004 of an inch n thick. Even with the best mechanical gauges the vary as much as 10%.

that a selection by mechanical gauges will not be accurate enough 'to guarantee the proper' thick-- 'ness and in addition does not take into account of evaporating variations in 4effective thickness due to unevenness might well exceed In additionv to this. thickness variation, the dielectric constant may It is evident, therefore,

the changes in dielectric constant.

The difficulty is emphasized in the manufacture of silver coated capacitors since the silver coating is in intimate contact with the mica and penetrates small irregularities in the surface of the mica. Mechanical gauges measure the thickest portion of the mica and do nottake into consideration small depressions which may actually increase the capacity considerably.

` 'The first expedient attempted was to make the silver coated mica capacitors of slightly higher capacity than the desired value, and to reduce the capacity to the desired value byV removingsome of the silver coating by a manual operation.

However, there were several disadvantages to this method of manufacture: (l) the cost was increased; (2) the dielectric strength was reduced due to mechanical damage. causing failure at high voltages; (3) the power factor increased: and (4) the adjustment range was limited.

In accordance with this invention, therefore, it is proposed to grade the mica in terms of actual capacity per unit area'by a system which takes into consideration not only the thickness and the dielectric constant but also small surface variations of the mica. A device for accomplishing this is shown in the figure of the drawing.

divided into an upper chamber 3 and a lower glass and is equipped with an extension arm l and a supporting member 9.

The lower chamber 5 is a s lall cylindrical cup whose upper end is open and lies in a horizontal plane. The upper chamber 3 is a similar cylindrical cup which is closed at the upper end and open at the lower end which lies in a horizontal plane parallel to, and adjacent the plane of the lower cup. The distance between the edges of the upper and lower chambers is approximately .005 of-an inch. The container i is filled with a fluid conductor I l such as mercury, or the like.

vchamber 5. .The container is preferably made of The lower chamber 5 is supported in a glass f worktable i3, the upper surface of the worktable being in the plane passing through the upper edge of the lower chamber 5. 4lior convenience, the.

the guide plate l5 extends outwardly and upwardly to guide the dielectric between the upper and lower chambers 3 and. 5, respectively. The surface tension of mercury is sufficiently .face of the mica.

' standard value.

great that there is substantially no leakage through the aperture between the uDDel and lower chambers. Even the repeated insertion of mica sheets through this aperture does not cause c an appreciable loss of mercury.

The extension arm I is provided to allow the mercury in the vcontainer to be replenished or removed for cleaning. An electrode I1 enters the upper chamber 3 through the extension army 1.

Electrical contact to the mercury in the lower' chamber 5 is provided by a screw I9 which extends into the cup and is connected to a contact rod 2| by means of which electrical contact may be made to an'external circuit.-

'Ihe .worktable I8 is securely mounted on a base 23 which supports a vertical supporting member 25. The upper chamber 3 is supported on the vertical supporting member 25 by an adjustable mechanism which includes a thumb screw 21 and a slidable supporting member 29 which engages the supporting member 9. The space between the upper and lower chambers may be suitably adjusted by means ofthe thumb' screw 27.

In using this device to Atest mica, it will be found that small flakes of mica are sometimes detached from the sheets. The presence of these flakes does not interfere with the operation of this device, however, since they immediately float tothe top surface of the mercury. It has been found that after several months of operation the mercury should be removed and cleaned to eliminate the dirt and mica which is collected on the and lower chambers is a function of the thickness and the dielectric constant of the mica, and is a measure of the ultimate capacity which may be expected when silver is evaporated on the surselect or grade each mica sheet according to its ultimate capacity. In this manner capacitors of a predetermined value may'be manufactured by selecting mica having a predetermined characteristic, or capacitors of approximately known values may he manufactured by grading the mica within any desired limits.

The measurement or indication of the capacity is accomplished by a device 33 which is not in itself a part of this invention and need not be described in detail. This indicator may be a capacity bridge if actual capacity values are desired. On the other hand, it may take the form of a device which automatically indicates relative capacity with respect to any predetermined The latter system would be used in grading mica for production purposes.

Manufacturing silver coated mica capacitors under the old methods produced a 40% yield of capacitors within the required limits. By grading the mica in accordance with the system set forth in this application, the yield of capacitors within a predetermined limit can be made as high as 90%.

Mica selected in accordance with this invention has also been utilized in the production of capacitors which use lead foil, and a definite im- It is, therefore, possible to provement has been noted. In fact, variations inthe nished capacity of such lead'foil capacitors has been found to be less than the originall tolerance of the mica. The device described in this application has many uses Aother than the particular one described. Due to its high speed it is useful in the test of changes in thickness or dielectric constants due to changes in the temperature of the mica, and may also be used to provide a rapid check on the uniformity of l dielectric strips', such as rolls of paper, by providing a. mechanism for pulling the paper through the device. Y i

I claiml as my invention: 1. In a device of the character described, a l container having a peripheral slit, a fluid in said container whose surface tension is sufllcient to prevent loss of said fluid through said slit, said slit being adapted to receive a sheet of ydielectric material which completely separates said fluid into two portions, each portion being in intimate contact with one surface of said dielectric, and means for making an electrical connection to A .each of said portions 'whereby the capacity be- I suillcient to prevent it from passing through the l space between said portions, said space being adapted to receive a thin sheet of dielectricwhose characteristics are to be tested, and means for maintaining said fluid in intimate contact against both sides of said dielectric within the limits of i said container.

3. In a device of the character described, a container which is divided into spaced upper and lower portions which respectively terminate in open ends which lie in closely adjacent horizontal 4 parallel planes, an electrically conducting fluid in said container' whose surface tension is suffi- .cient to prevent said fluid from passing through the space between said portions, said space being i adapted to receive a thin sheet of dielectric whose characteristics areto be tested, said dielectric completely dividing said fluid into two insulated portions, and means for maintaining said fluid in intimate contact with opposite sides of said dielectric throughout th tainer. f

4. In a device for testing the characteristics of a dielectric, a container which is divided into closely spaced upper and lower chambers, .the

adjacent ends of which `are open and terminate l in parallel planes and the outer ends of which are closed, an electrically conducting fluid in said container whose surface tension is sufficient to prevent said fluid from passing through the space between said upper and Vlower chambers, u

a worktable whose upper surface lies in the plane of the open end of said lower chamber, and an annular guide plate surrounding the open end of said upper chamber adapted to guide a sheet of dielectric which is to be tested between the open ends of said upper and lower portions, thereby separating said fluid into two insulated portions. r l

5. A device of the character described in claim 4 in whichthe spacing between said upper and u lower chambers is approximately five thousandths of an inch.

ALLAN R. OGILVIE.

area defined by said con- 

